1. Field of the Invention
This invention is in the field of non-stationary clutter cancellation for use with radar imaging and/or target detection.
2. Description of the Related Art
Synthetic Aperture Radar (SAR) radar is used for ground mapping as well as target identification. The general principle behind SAR is to coherently combine the amplitude and phase information of radar returns from a plurality of sequentially transmitted pulses from a relatively small antenna on a moving platform.
The plurality of returns creating a SAR image generated by the transmitted pulses along a known path of the platform make up an array. During the array, amplitude as well as phase information returned from each of the pulses, for each of many range bins, is preserved. The SAR image is formed from the coherent combination of the amplitude and phase of return(s) within each range bin, motion compensated for spatial displacement of the moving platform during the acquisition of the returns for the duration of the array.
The plurality of pulses transmitted during a SAR array, when coherently combined and processed, result in image quality comparable to a longer antenna, corresponding approximately to the “length” traveled by the antenna during the array.
However, where ground targets of interest are near a source of fast varying clutter, such as foliage on vegetation subject to turbulent wind, SAR imaging may not reliably detect or image the targets. Besides the desired target signal reflected from the target of interest, there are also returns from fast varying clutter, slowly varying clutter, and noise. In the prior at, lower operating frequencies (VHF and UHF) have been used to penetrate forest canopies. This is because VHF and UHF frequencies have lower attenuation through forest canopies or other vegetative cover, while still providing a radar return from metal, reflecting targets. However, this VHF and UHF frequency approach failed in many instances because of the effect of the Doppler shift induced in the SAR imaging by slow and fast clutter coupled to the presence of noise.
The fast varying nature of the clutter from vegetation modulates the Rayleigh distributed component of clutter, making the clutter non-stationary. This introduces uncertainty as to the Doppler of the clutter. Current Space Time Adaptive Algorithm (STAP) algorithms for clutter cancellation do not provide sufficient clutter rejection to allow imaging/detection in non-stationary clutter.
Similarly, in a maritime environment, radar returns from sea surface targets are obscured by fast varying clutter returns from small ocean waves and surf, as well as slow varying clutter from larger waves. Generally X band radar is used for maritime applications. As above, even with X band operation, the fast varying clutter modulates the slow varying, Rayleigh distributed component of clutter obscuring the target of interest.
Detection of slow moving targets within non-stationary clutter such as wind blown foliage or sea clutter is degraded because there is no definite correspondence between target angle and the Doppler shift of the clutter.